The present invention relates to liquid level measurement and control using temperature-sensitive sensors, and more particularly to a computer-aided system and associated technique for accurately monitoring and automatically controlling liquid levels in a tank despite variations in tank temperature.
Semiconductor thermistor elements are commonly employed as temperature-sensitive sensors in a variety of liquid level control systems designed to indicate and regulate the amount of liquid present in a tank or reservoir. In such control systems, the thermistor elements, which change their electrical resistance with absolute temperature, are typically mounted within the tank or reservoir at a certain predetermined height above the bottom of the tank and at a certain preconditioned temperature unexposed to the liquid. Generally, thermistors having negative temperature coefficients of resistance are employed and heated with applied current to initially reduce their resistance. If the liquid reaches the predetermined level in the tank and the thermistor becomes submerged, the thermistor will be cooled because of the greater thermal conductivity of the liquid and the thermistor resistance will increase. The resulting voltage increases across the thermistor is typically detected by a sensor circuit and used either to provide an overflow alarm signal or to control pumps and valves that will halt or reverse the liquid flow.
While thermistor control systems have been generally successful in liquid level monitoring and measurement, such systems have experienced a critical problem in their application to certain types of automated processing involving the control of liquid levels within tanks that are themselves heated to maintain the liquid at an elevated temperature. In such automated processing applications, frequently involving medical laboratory testing, increases in the tank temperature are unavoidably sensed by the thermistors and effectively offset the cooling effect of the rising liquid contacting the thermistor so that there is little or no change in the absolute temperature of the thermistor. As a result, there is no indication of an actual liquid level attainment and a dangerous and damaging overflow may occur. This problem of liquid overflow from heated tanks has been particularly evident with liquids of relatively low thermal conductivity, such as liquid wax.